Further studies of the continuous UV emission produced by electron impact on CF4

Abstract

Abstract. The intense continuous UV emission which extends from 200 nm to beyond 500 nm produced by electron impact on carbontetrafluoride, CF4, has been investigated in a crossed electron-beam -gas-beam apparatus as well as in a gas-target apparatus under single collision conditions with special emphasis on the regime of impact energies below 50 eV. Systematic measurements of excitation functions (relative emission cross sections) at various wavelengths between 225 nm and 420 nm revealed the same onset of 23.5 + 2 eV and the same energy dependence for all excitation functions independent of the selected wavelength indicating that the continuous emission consists of a single fluorescence contribution. Evidence of a second onset around 40 eV, roughly 16 eV above the first onset which is an energy close to the ionization energy of atomic fluorine (17.4 eV), was foundmore or less prominent -in all excitation functions. The presence of the second onset lends strong support to the assignment of the excited (CF3 +)* fragment ion as the emitting source of the emission continuum. We identify the lower onset at 23.5 eV with the electron-impact induced breakup of the parent CF4 molecule into an excited (CF 3 +)* ion and a ground state fluorine atom ("neutral" channel), whereas the second onset around 40 eV indicates the presence of an "ionic" fragmentation channel in which an excited (CF 3 +)* fragment ion is produced together with a fluorine ion, F +. Wavelength scans taken at impact energies above and below the second onset revealed no significant difference in the envelope of the observed emission contiuum which further supports the notion that the emission continuum consists of a single fluorescence contribution.

Electronic properties and primary dissociation channels of fluoromethane compounds

“…21) The excited-state energies and the emission band from the excited states of the CF 3 radical have not been fully discussed and remain an ambiguous subject. The calculated excitedstate energy of D 3h CF 3 *(1A 2 ″) obtained by using EOMCCSD/aug-cc-pVTZ + 4s + 4p after MP2/aug-cc-pVDZ optimization is 2.9 eV higher than the D 3h CF 3 ground-state energy and corresponds to a part of a mysterious emission band (200-500 nm with peak energy of 285 nm) observed by Müller et al 22) (see also Fig. 6 and Fig.…”

“…The arrows at the right-hand side correspond to the mysterious emission band. 22) variations are also shown. The ionization potential (red line) crossed the 2E and 3E (blue lines) excited-state potentials along the deformation from a C 3v to a D 3h structure, so that excitation to the higher vibrational excited states of 2E and 3E took place and then led to auto-ionization.…”

Absolute and relative emission cross sections for electron impact on CH3F, CH2F2 and CHF3

“…Optical emission spectra and absolute and relative emission cross sections following electron impact on molecules were measured by two different techniques which are described in more detail earlier [12,13]. In a crossed beam apparatus, the electron beam interacts with a molecular beam that emanates from a multicapilary orifice into the UHV-vacuum recipient.…”

“…The shift could be caused by three different factors: carbon reacting with the heated filament, reactions of fluorine atoms with the heated filament and surface deposits of other components. These factors have been examined systematically earlier [13]. In order to measure the contact potential during the determination of relative cross sections in the gas target system, all examined gases were mixed with N .…”

Ionization by Drift and Ambipolar Electric Fields in Electronegative Capacitive Radio Frequency Plasmas

“…We analyze the electron dynamics in CF 4 discharges with a gap of 1.5 cm, driven at 13.56 MHz, at different pressures and driving voltages. In the experiment, the plasma emission at 250 nm originating from dissociative deexcitation of excited CF þ 3 ions created by electron impact on CF 4 [18] is detected space resolved between the electrodes and time resolved within the rf period by PROES [3,17]. The plasma is confined between the electrodes by a fused silica cylinder to improve the discharge symmetry.…”